The present invention relates to a method for producing a glass preform for use in the fabrication of optical fibers. More particularly, the invention is concerned with a method for producing a quartz glass preform with fluorine incorporated therein.
A glass preform for fabrication of optical fibers is composed of a core and a cladding surrounding the core. The core must have a higher refractive index than the cladding so as to allow easy propagatin of light therethrough.
In order to increase the refractive index of the core, additives such as TiO.sub.2, GeO.sub.2 and Al.sub.2 O.sub.3 are usually added (for a base material of silica). In general with optical fibers, pure quartz glass is often used to form the cladding. In this case, pure quartz glass is produced so that n=1.4585 and .DELTA.n=0.
Referring to Figs. 1A and 1B, there are shown diagrams illustrating the distribution of the refractive index for optical fibers. A and B in these figures indicate, respectively, the core and cladding. The difference in refractive index between the core and cladding is usually indicated in terms of a relative refractive index difference (in percent). That is, assuming that the refractive indices of the core and cladding are n.sub.1 and n.sub.2, respectively, the relative refractive index difference .DELTA.n.sub.12 % is represented by the following equation: ##EQU1##
FIG. 1A shows the general distribution of refractive index for single mode optical fibers. In this case, .DELTA.n.sub.12 is usually 0.3 to 0.5%. FIG. 1B shows the general distribution of refractive index for multi-mode optical fibers. For optical fibers for ordinary communication purposes, .DELTA.n.sub.12 is usually about 1%, and for large aperture optical fibers used in computer ring communication applications, .DELTA.n.sub.12 is usually about 2 to 4%.
Oxide additives such as GeO.sub.2 added to increase refractive index cause light scattering (Rayleigh scattering) because of their intrinsic characteristics. As the amount of the additive added is increased, the degree of light scattering (Rayleigh scattering) due to the additive increases. This is not desirable for light transmission.
If the additive is added in large amounts, formation of bubbles and a crystal phase in the base material results. In the case of GeO.sub.2, for example, the formation of GeO gas easily occurs, thereby producing bubbles. In the case of Al.sub.2 O.sub.3, clusters of Al.sub.2 O.sub.3 crystals are easily formed. This is not desirable for light transmission characteristics and also for the strength of the final optical fibers. Furthermore, the coefficient of thermal expansion of glass increases, which makes the glass preform fragile. Therefore, also from the viewpoint of light propagation and glass strength, it is preferred that the amount of the additive added to the glass preform to be reduced.
For this reason, there is generally employed a procedure in which the refractive index difference is increased by lowering the refractive index of the cladding. For example, additives which lower the refractive index, such as B.sub.2 O.sub.3, fluorine or a combination thereof, can be added to the cladding. B.sub.2 O.sub.3, however, has disadvantages in that the coefficient of thermal expansion of the resulting cladding greatly changes with the concentration of B.sub.2 O.sub.3 and that the refractive index changes upon heating. Furthermore, with regard to light propagation characteristics, the cladding has an absorption loss due to B.sub.2 O.sub.3 in the longer wavelength region. Thus, it is preferred to use fluorine as a refractive index-lowering agent.
It is known that the addition of fluorine to quartz glass enables the production of optical fibers of various refractive index distributions, and that, by the proper choice of structure, there can be obtained optical fibers of low dispersion over a wide wavelength region.
The advantage that can be obtained by using fluorine as an additive is that, since the refractive index of the cladding can be made lower than that of pure quartz, pure quartz or quartz glass with a small amount of additive added thereto can be used in the fabrication of the core.
FIGS. 2A through 2D show typical refractive index distribution structures of which those of FIGS. 2A and 2C are of the step index type and those of FIGS. 2B and 2D are of the graded index type. In all of FIGS. 2A to 2D, fluorine is added to the cladding. With regards to the core, in the case of FIG. 2A, small amounts of oxides which increase the refractive index, such as GeO.sub.2 and P.sub.2 O.sub.5, are added to quartz glass, whereas in the case of FIG. 2C, high purity quartz glass containing no additives is used. In FIG. 2B, the amount of fluorine added is decreased continuously from the periphery of the core to the center, and the central portion is made of pure quartz glass not containing fluorine (the refractive index of pure quartz glass is n=1.4585, .DELTA.n=0). In FIG. 2D, the amount of fluorine added is decreased continuously from the periphery of the core to the center, and at a certain distance from the periphery, starts the addition of an additive used to increase the refractive index of quartz glass, with the amount of the additive added increasing continuously towards the center.
As a matter of course, to control the refractive index and facilitate the working of the glass, additives such as GeO.sub.2, P.sub.2 O.sub.5, B.sub.2 O.sub.3 and Al.sub.2 O.sub.3 can be used in combination with fluorine in the cladding and core.
In order to obtain the same refractive index difference as shown in FIG. 1 for optical fibers of quartz glass with fluoride added thereto, it is sufficient to decrease the amount of oxides added to the core, or alternatively, not to add the oxides at all. This leads to a reduction in the degree of Rayleigh scattering due to the presence of the additive. Thus, the rresulting optical fiber is preferred as a waveguide. Fluorine is available in abundance as compared with additives such as GeO.sub.2, and furthermore is advantageous from an economical standpoint in that its purification is easy. Another feature is that a fluorine-based compound fas is superior not only as a starting material for the additive, but also as a dehydrating agent for removing water contained in the soot.
Various techniques are known for fabrication of quartz glass optical fibers, including the inside chemical vapor deposition (CVD) method (Japanese patent publication Nos. 23186/76 and 22423/80), the outside chemical vapor deposition (CVD) method (Japanese patent Kokai publication (unexamined) No. 10055/74), the vapor axial deposition (VAD) methods (Japanese patent Kokai publication (unexamined) No. 71316/76), and the plasma chemical vapor deposition (CVD) method (Japanese patent Kokai publication (unexamined) No. 54446/76). Of these methods, the outside CVD method utilizing a flame hydrolysis reaction and the VAD method are superior in productivity and are economical procedures. On the other hand, although fluorine can be added to quartz glass by a procedure utilizing a flame hydrolysis reaction, it is quite difficult to uniformly add a sufficient amount of fluorine to the quartz glass by this procedure.
Japanese patent Kokai publication (unexamined) No. 15682/80, for example, discloses a method of incorporating fluorine into a glass preform. In accordance with this method, however, a variation in the refractive index due to the addition of fluorine as determined relative to that .DELTA.n.sub.12 of quartz glass is at most about -0.2%. That is, the method has a disadvantage in that the amount of fluorine that can be added is limited.
Fluorine is incorporated into fine particles of quartz according to the following equation: EQU SiF.sub.4 (g)+3SiO.sub.2 (s).fwdarw.4SiO.sub.1.5 F(s), (1)
where (g) and (s) indicate gas and a solid. Since, however, water resulting from combustion is present in the oxyhydrogen flame used in the production of soots, SiF.sub.4 (g) reacts with water as represented by the following equation: EQU SiF.sub.4 (g)+2H.sub.2 O(g).fwdarw.SiO.sub.2 (s)+4HF(g) (2)
That is, SiF.sub.4 is consumed upon reaction with a large amount of water present in the flame as well as acting as an additive for quartz glass. It is apparent, therefore, that the efficiency of addition of SiF.sub.4 drops.
Japanese patent publication No. 15682/80 discloses a method in which fluorine is incorporated into glass by applying a fluorine-based compound gas at the step of synthesizing glass in gas phase. This method does permit the incorporation of fluorine into glass, but has a disadvantage in that the efficiency of deposition of glass and the yield of incorporation of fluorine (doping yield) are low. The reason for this is considered that, in the flame hydrolysis method using a H.sub.2 /O.sub.2 flame, water in the flame reacts with a fluorine-based compound gas (e.g., SF.sub.6) according to equation (3) below, thereby producing HF gas: EQU SF.sub.6 +3H.sub.2 O.fwdarw.SO.sub.3 +6HF. (3)
HF gas is stable, and almost all of the fluorine-based compound gas is converted into HF gas at elevated temperatures as long as there is water present. Thus, a minor proportion of fluorine-based compound gas is utilized as the dopant starting material.
Hydrogen fluoride (HF) formed in the reaction specified by equations (2) and (3) acts to corrode glass, particularly quartz (SiO.sub.2), and easily reacts with fine quartz particles formed in the flame, as shown by the following equations (4) and (5): EQU SiO.sub.2 (s)+2HF(g).fwdarw.SiOF.sub.2 (g)+H.sub.2 O(g) (4) EQU SiO.sub.2 (s)+4HF(g).fwdarw.SiF.sub.4 (g)+2H.sub.2 O(g) (5)
This inhibits the grain growth of glass particles and decreases the amount of fine glass particles being deposited. This is apparent from the fact that, as the amount of a fluorine compound added is increased, the rate of deposition of fine glass particles drops, and finally they do not deposit at all.
Japanese patent Kokai publication (unexamined) No. 7533/80 discloses a method which is intended to overcome the above-described problems of the method of Japanese Patent Publication No. 15682/80. Specifically, it discloses: (1) a method for producing a glass material for optical glass particles formed by the flame hydrolysis method in an atmosphere of a fluorine compound gas at 1,000.degree. C. or less, and thereafter sintering the laminated body by heating it to more than 1,400.degree. C. in an inert gas atmosphere; and (2) a method for producing a glass material for optical transmission which comprises heating the glass particle laminated body of (1) in a fluorine compound/inert gas atmosphere to more than 1,400.degree. C. to form a glass material containing fluorine. Methods (1) and (2) enable fluorine to be incorporated more effectively than the method of Japanese patent publication No. 15682/80. It has been discovered, however, that methods (1) and (2) still have disadvantages as described below.
In method (1), the rate of incorporation of fluorine into glass is slow and, in some cases, the ultimate optical fibers contain impurities such as Cu and Fe, and an increase in transmission loss due to such impurities reaches about 3 to 5 dB/km at a wavelength of 1.30 micrometer (the usual loss at this wavelength band is 0.3 dB/km). The amount of fluorine incorporated into glass by method (1) is -0.20% in terms of the refractive index difference .DELTA.n.sub.12 (F).
It has been discovered that, in some cases, the final optical fibers contain Cu and Fe. It is known that Cu and Fe cause an absorption loss, which is responsible for an increase in the transmission loss.
Method (2) is an efficient procedure in that, as compared with method (1), the rate of incorporation of fluorine is high and the amount of fluorine added is large. After a processing time of 6 hours, .DELTA.n.sub.12 (F) reaches -0.25%. However, the obtained glass preform is seriously corroded and has an irregular surface. The core tube used in the production of the glass preform, which is a quartz muffle tube used to hold therein a gas atmosphere, is seriously corroded and, in some cases, perforations are formed in the walls of the tube. This etching is considered to partly accelerate the incorporation of impurities from the muffle tube into the soot preform. The loss of the thus-produced optical fiber is about 10 dB/km at a wavelength of 1.30 micrometer. Since the OH group content of the optical fiber is 0.05 ppm or less, it cannot be considered that the increase in absorption loss at 1.30 micrometer is due to the presence of the OH group. There are many experiments supporting the conclusion that the increase in absorption loss due to impurities such as Cu and Fe existing in the optical fiber amounts to 9.5 dB/km.
In addition, the optical fiber produced by the above-described method has disadvantages in that the absorption loss due to the OH group changes with time, and as the temperature rises, the absorption loss considerably increases.
One of the reasons why impurities such as Fe and Cu are present in the optical fiber is that corrosion of the core tube allows Fe.sub.2 O.sub.3 and CuO present in the core tube walls to migrate to the surface of the tube and to intermingle with the soot, undergoing reactions represented by the following equations: EQU Fe.sub.2 O.sub.3 +2F.sub.2 .fwdarw.2FeF.sub.2 +3/2O.sub.2 ( 6) EQU CuO+1/2F.sub.2 .fwdarw.CuF+1/2O.sub.2 ( 7)
Although FeF.sub.2 and CuF are solid up to 1,100.degree. C., they sublimate at temperatures in excess of 1,100.degree. C., thereby intermingling with the soot. Thus the soot preform is contaminated with FeF.sub.2 and CuF.
When Fe.sub.2 O.sub.3 and CuO are contained in the soot preform, even if they undergo the reactions of equations (6) and (7), the resulting products FeF.sub.2 and CuF are not removed from the soot and remain therein as impurities since they are solid at temperatures below 1,100.degree. C. Thus, in accordance with either of methods (1) and (2), impurities are left in the optical fibers.